1. Field of the Invention
This invention relates to a marker signal detecting system, and more particularly, to a system which detects a marker signal periodically inserted in an information signal such as a scrambled audio signal or the like.
2. Description of the Prior Art
In a scrambling system for an audio signal, an information signal is divided into a plurality of blocks, with each block then being formed of a plurality of segments. The segments of the audio signal are re-arranged on a timebase in a predetermined order to form the scrambled audio signal. On reception, the segments are rearranged into the original order to reproduce or restore the original audio signal.
If the scrambled audio signal is subject to timebase fluctuations which can occur, for example, if the signal is first recorded by a video tape recorder and then played back, the original audio signal reproduced from the video tape recorder can be distorted. Alternatively, noise can be superimposed on the signal, since a connecting portion between respective segments is displaced due to the timebase fluctuations, thus causing significant deterioration in the quality of the audio signal.
In a proposed system, a marker signal has been superimposed upon the audio signal to indicate or mark the beginning of each segment in a block of the audio signal in order to deal with the errors which occur from timebase fluctuations. A notch filter or the like is used to extract the marker signal from the information signal. A phase-locked loop (PLL) circuit can be used to lock in on the extracted marker signal which repeats at periodic intervals. However, in such a system, the reproduced signal is theoretically worse than the original signal because the marker signal is inserted within a band or region of the audio signal so that a portion of the audio signal is obliterated. An additional drawback to this system is its expense in that the notch filter used to extract the marker signal is expensive.
In order to deal with such problems, it has also been proposed to utilize as the marker signal a high or low frequency signal which is outside the band or region of the audio signal. When a high frequency signal is used as the marker signal, however, the signal transmission path is restricted. When a low frequency signal is utilized as the marker signal, a long time is needed to detect the signal. In addition, a low frequency signal is difficult to detect.